# 3.5.1.3.41 Incgamma

## Definition

This function is used to calculate the incomplete Gamma function at x, with parameter a, which is denoted by: $incgamma\left( a, x\right)=P\left( a, x\right)=\frac{1}{\Gamma \left( a\right)}\int _0^x t^{a-1}e^{-t}\,dt$

where $\Gamma (a)$ is the value of Gamma function at a. The parameters, a and x satisfy: a > 0 and x ≥ 0.

## Syntax

double incgamma(double a, double x)

## Parameters

a (input, double)

The argument a of the function. Constraint: a > 0.

x (input, double)

The argument x of the function. Constraint: x ≥ 0.

## Return

Returns the value of the incomplete Gamma function at x, with parameter a.

## Example

aa = incgamma(1, 5);
aa = ; //0.99326205300093